The present invention relates to dampening systems for offset lithographic printing presses, and more particularly, to an improvement in the dampening solution feed apparatus which is disclosed in U.S. Pat. No. 4,455,938.
The major components of a typical offset press are the plate cylinder, blanket cylinder, dampening system, and inking system. Secured around the plate cylinder is a planographic printing plate upon which is imposed an image defined by a gradation of oleophilic dots or areas and a hydrophilic background, the intensity of the image being directly proportional to the concentration of such dots and/or the shape of such areas.
The function of the dampening system is to thoroughly moisten only the background of the plate with a substantially aqueous solution. Grease-based ink is applied to the plate and adheres only to the oleophilic dots to create an inked image which is transferred to an adjacent blanket cylinder and subsequently to the paper to be printed. Prior to the invention disclosed in said U.S. Pat. No. 4,455,938, the most common dampening devices used in offset lithographic presses were either of the type disclosed in U.S. Pat. No. 4,088,074 issued 9 May 1978, or of the oscillating ductor roller type which has been used in this art for many years.
The dampening system is a critical element of the press. If the quantity and application of the dampening solution is not precise, the solution and ink will incorrectly coat the respective hydrophilic and oleophilic areas, and the finally printed image will be distorted. Conventional ductor roller dampening systems control the amount of solution transferred to the form roller, which contacts the plate cylinder, by adjusting the rate of oscillations of the ductor roller which reciprocates back and forth from a solution reservoir to the train of dampening rollers.
The amount of solution which is then actually transferred to the plate is controlled by adjusting the pressure between the form roller and the plate cylinder. Excess solution is nipped between the form roller and the plate cylinder and accumulates on the form roller. This excess solution must eventually be reduced by decreasing the number of oscillations of the ductor roller, or it will accumulate enough to result in over-moistening of the printing plate. This is accomplished by the dampening apparatus of U.S. Pat. No. 4,455,938, and as a result it has enjoyed considerable success as a replacement or retrofit dampener for a number of small printing presses marketed by various companies. Depending upon the existing press structure, the dampener may take various configurations so as to adapt to and receive its drive from such presses.
While dampeners of the type disclosed in said U.S. Pat. No. 4,455,938 are useful with many sizes and types of lithographic presses, these dampeners in a 11 inch by 17 inch (279 mm. by 432 mm.) sheet-fed presses which are used in small to medium size printing establishments, often called "job shops" in the trade. Those shops may also have need for an over-coating (e.g. varnishing) function, using water-based transparent coatings to enhance the appearance of a printed page. Special attachments to such presses, for coating the back side of sheets via the impression cylinder, are known. One such attachment is disclosed in published German patent application DE No. 36 14 704 Al.
The possibility of using a dampener, alternatively, as a coater, represents considerable savings in equipment investment to such shops, which often have budget restraints.